ACPI: fix broken usage of name.ascii

This issue was fixed indirectly in mainline by commit
0175d562a29ad052c510782c7e76bc63d5155b9b.

acpi_namespace_node's name.ascii field is four chars, and not NULL-
terminated except by pure luck.  So, it cannot be used by sscanf() without
a length restriction.

This is the minimal fix for both stable 2.6.27 and 2.6.28.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Lin Ming <ming.m.lin@intel.com>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>